European patent EP 1 579 728 B1 discloses a hearing aid with two microphones, wherein output signals from both microphones are combined to provide directional microphone signals.
Combining signals from two or more microphones in a hearing device is often encountered in the prior art. A prerequisite for obtaining e.g. a good “figure-eight” directional microphone signal is that the frequency characteristics of the microphones match each other closely. However, the physical embedding of a microphone or electroacoustic transducer affects its frequency characteristic. Therefore, such transducers are typically embedded in equal physical environments within the hearing-device housing and with conduits of equal length leading from respective sound inlets in the housing to the respective transducers. Since the locations of the sound inlets are typically dictated by audiologic requirements, this puts undesired constraints on the physical layout of the hearing device.
U.S. Pat. No. 3,458,668 A discloses a hearing aid with two microphones, each with a conduit leading from the microphone to a respective opening in the hearing-aid housing. The conduits have different lengths. The amplitude of the sound signal reaching a microphone may be changed by changing the length of the respective conduits. In this configuration, the frequency characteristics of the microphones do generally not match each other.
US patent application 2008/013770 A discloses a microphone array with guide tubes of different lengths each leading from a respective microphone to a respective opening in the housing. A damper is placed in the shorter ones of the guide tubes to provide equal sound signal delays between the openings and the microphones. Also in this configuration, the frequency characteristics of the microphones do generally not match each other.
International patent application WO 2004/098232 A1 discloses a hearing aid with a microphone having a first tube leading sound to the microphone. In order to prevent ultrasonic sound from reaching the microphone, a second tube is connected to the first tube near the microphone. The length of the second tube is dimensioned to have the second tube function as a quarter-wavelength resonator that dampens ultrasonic frequencies. Applying these teachings to a hearing device with two microphones would constrain the physical layout of the hearing device further.
It is an object of the present invention to provide a hearing device, which does not suffer from the above problems. It is a further object of the present invention to provide a hearing device with two or more microphone units each having a conduit leading from a respective sound inlet in the hearing-device housing to a respective transducer, wherein the lengths of the conduits may differ without causing a difference in the frequency characteristics of the microphone units and wherein ultrasonic frequencies may be dampened, while at the same time allowing a higher freedom in the physical layout of the hearing device.